I. Field
The present invention relates generally to communication, and more specifically to techniques for reducing the likelihood of missing pages during cell reselection in a wireless communication system.
II. Background
A terminal in a wireless communication system (e.g., a cellular phone in a cellular system) is typically designed to operate in one of several modes, such as active and idle, at any given moment. In the active mode, the terminal can actively exchange data with one or more base stations in the system (e.g., for a voice or data call). In the idle mode, which is also referred to as standby mode, the terminal typically monitors a paging channel for messages alerting the terminal to the presence of incoming calls.
In a Global System for Mobile Communications (GSM) system, a terminal that is just powered on or has just lost coverage initially searches for “suitable” cells from which it can receive service. A “cell” can refer to a base station in the system and/or the coverage area of the base station, depending on the context in which the term is used. A suitable cell is one that meets a set of criteria defined by GSM. If a suitable cell is found, then the terminal performs registration with the cell as necessary. The terminal then “camps” on the cell if it is in the idle mode. While camped on the cell, the terminal performs tasks as specified by GSM so that it can (1) receive system information from the cell, (2) receive paging messages from the cell (e.g., alerting the terminal to incoming calls), and (3) initiate call setup for outgoing calls or other actions. The cell on which the terminal is camped is referred to as the “serving cell.”
While camped on the cell, the terminal periodically checks to see if there is a better cell that it can camp on and receive service (e.g., another cell with a higher received signal level). The terminal may also select another serving cell if, for example, the current serving cell has become barred, the terminal cannot receive the signal from the current serving cell because the channel condition has degraded, and so on. The process of selecting another serving cell is referred to as “cell reselection.” The terminal performs cell reselection while in the idle mode so that it can monitor the system for incoming paging messages and initiate a call even if the channel condition changes (e.g., if the terminal moves to a new location).
A key challenge in a GSM system (and in other cellular systems) is performing cell reselection while in the idle mode without missing an incoming call. In one conventional method of performing cell reselection in GSM, a terminal (1) obtains the necessary measurements and determines that a reselection to a new cell is needed, (2) switches to the new cell and gathers all of the required system information for the new cell, and then (3) starts receiving paging messages from the new cell. For this method, the terminal receives paging messages from the old serving cell up to the time the reselection decision is made, and starts receiving paging messages from the new cell after it has gathered all of the required system information. The terminal does not receive paging messages during the time that it is gathering system information for the new cell, which may take from approximately one to ten seconds in GSM. The terminal would then miss any page messages sent to it during this time period, which is highly undesirable.
There is therefore a need in the art for techniques to reduce the likelihood of missing paging messages during cell reselection.